Solar Envelope Research Articles

Models of Saturn's interior: Evidence for phase separation

Models of Saturn's interior have been constructed based on an accumulation picture of planet formation. It was found that central pressures were ∼90 Mb, and central temperatures ∼10 4°K. In sharp contrast to Jupiter, which requires large amounts of heavy material in the envelope to match the observed gravitational quadrupole moment, Saturn requires an almost solar envelope. Indeed, the ratio of enhanced material in the envelope to material in the core is less than ∼0.1, while the corresponding value for Jupiter is ∼2.

The sensitivity of nonradial P mode eigenfrequencies to solar envelope structure

Eigenfrequencies are calculated for nonradial p mode oscillations in the sun. These frequencies are shown to depend on the adiabat of the solar convective envelope. The frequency dependence is greatest for the p2 and p3 modes described by spherical harmonics with l = 500 to 1000. The solar envelope model includes a realistic representation of the chromosphere out to the base of the corona. For the chromospheric model of Vernazza, Avrett, and Loeser (1976) chromospheric modes with a distinct locus of eigenfrequencies exist near an eigenfrequency of 0.026 Hz. The actual frequencies of these eigenmodes depend on the thickness of the chromosphere. Because of the difference in the dependence of eigenfrequency on l for the chromospheric modes as compared with the p modes, the two types of mode can be distinguished observationally. If detected, the chromospheric modes would provide a good way of determining the thickness of the chromosphere.

Observations of nonradial p-mode oscillations on the sun

Observations of the solar velocity field with a diode array attached to the echelle spectrograph of a vacuum tower telescope are described which resolve the solar five-minute oscillatory motion into distinct bands of power. Previous observations are discussed which show that the solar five-minute oscillations can be resolved into frequencies having the character of nonradial p-mode eigenfrequencies of the solar envelope. The present observations confirm the observed frequencies and sharpen the previous resolution of the five-minute oscillations into ridges on the (wavenumber, frequency) plane. A comparison with earlier calculations indicates that the theoretical frequencies are in good but not perfect agreement with those observed. It is concluded that the identification of the five-minute oscillations as nonradial p-mode oscillations in the solar envelope is established beyond doubt.

On the nature of photospheric magnetic fields beneath large coronal holes

We consider proposed mechanisms for the formation of coronal holes, and identify as crucial the issue whether the holes are permeated by rigidly rotating fields. It is suggested that the interaction between such a field and the differentially rotating, diffusive solar envelope will produce a fore aft asymmetry in the distribution of fields which emerge to the photosphere. An initial study is carried out in the context of an illustrative example, and the results indicate that the asymmetry may be observed for a certain range of parameters involving the properties of the solar envelope and the characteristic size of the emerging field pattern.

Evidence for extended solar wind heating by fast hydromagnetic waves

In order to reduce the discrepancy between solar wind measurements at 1 AU and the corresponding predictions of the basic two-fluid model a nonthermal heating of solar wind protons in the outer corona up to distances of about 0.1 AU from the sun was postulated. In the absence of thermal energy transfer to an anisotropic plasma the quantities c∥ = T∥B²/n² and c⊥ = T⊥/B (where T∥ and T⊥ denote proton temperature components parallel and perpendicular to the magnetic field with intensity B and n denotes the proton number density) constitute adiabatic invariants. Accepting the hypothesis that damping of fast-mode hydromagnetic waves near the sun strongly dominates the heating and acceleration of the solar wind, we argue that c∥ is increased thereby. Depending on the coupling between parallel and perpendicular degrees of freedom in the turbulent solar envelope, c⊥ will be raised to some extent too. If we may neglect interplanetary irreversible processes, these values are transported to the earth and may give some direct information on the turbulent outer corona. We therefore invoke c⊥ as a new natural ordering parameter instead of the customarily employed bulk velocity in a statistical presentation of solar wind data measured on the earth-orbiting satellite Heos 2. This choice reveals completely novel features that have been missed in the conventional presentation. We interpret these features as a signature of an extended heating process which survives the transit of the solar wind plasma to 1 AU. We note that the purely wave-heated two-fluid model of Barnes and co-workers was unable to reach flow velocities exceeding 450 km/s. Our data consistently indicate that for larger flow velocities additional heating and acceleration processes (e.g., nonspherically symmetric outflow from coronal holes) come into play. We suggest that a suitable combination of a corotating model and the wave-heated two-fluid model should be able to predict both the observed macroscale relation between proton temperature and bulk velocity and the dynamic effects in the moderately active solar wind. Further, we propose that proton double streams and related deformations of the proton distribution functions might be a relic of the interaction with a fast-mode hydromagnetic wave field which propagates from the inner corona.

Outward transport of angular momentum by gas convection and the equatorial acceleration of the Sun

A new mechanism for sustaining enhanced rotational velocity of the outer layers of the solar convective envelope is considered.

Solar seismology. I - The stability of the solar p-modes

We investigate the stability of the radial p-modes of the Sun by computing nonadiabatic eigen-values and e1genfunctions for a solar envelope model which extends from an inner radius r ≈ 0.3 R_☉ out to an optical depth r ≈ 3 x 10^(-4). Our calculations take into account in a crude fashion the response of the convective flux to the oscillation. The dynamical effect of turbulence in the convection zone is parametrized in terms of a turbulent shear viscosity. The results of our calculations are as follows. If damping by turbulent viscosity is neglected, all modes with penriods longer than 6 minutes are unstable. The familiar K-mechanism, which operates in the H ionization-H^- opacity region, is the dominant source of driving of the oscillations. Modes with periods shorter than 6 minutes are stabilized by radiative damping in the solar atmosphere. When turbulent dissipation of pulsational energy is included, all modes are predicted to be stable. However, the margin of stability is very small. In view of the large uncertainty that must be assigned to our estimate of turbulent damping, we conclude that theoretical calculations cannot unequivocally resolve the question of the stability of the solar p-modes.

A model for the origin of solar wind stream interfaces

The basic variations in solar wind properties that have been observed at 'stream interfaces' near 1 AU are explained by a gas dynamic model in which a radially propagating stream, produced by a temperature variation in the solar envelope, steepens nonlinearly while moving through interplanetary space. The region thus identified with the stream interface separates the ambient solar wind from the fresh hot material originally in the stream. However, the interface regions given by the present model are thicker than most stream interfaces observed in the solar wind, a fact suggesting that some additional physical process may be important in determining that thickness. Variations in the density, speed, or Alfven pressure alone appear not to produce streams with such an interface.

Interplanetary stream interfaces

At l AU there is a distinct boundary (the stream interface) at the leading edge of a stream in the solar wind, characterized by an abrupt drop in density, a similar increase in temperature and a small increase in speed. It is suggested that stream interfaces form in the interplanetary medium as a consequence of the non-linear evolution of streams generated by an increase in temperature in the solar envelope. This evolution eventually leads to the formation of a reverse shock behind the interface and a forward shock ahead of it. Two instances in which both a stream interface and a reverse shock had developed at l AU are presented. Examples of flare generated shocks which passed through a stream and were observed near a stream interface are also presented. It is shown that stream interfaces are definitely not the same structures as piston boundaries. It is noted that slow shocks, like stream interfaces, always occur ahead of streams and may develop in the interplanetary medium.

On the generation of high-energy particles in solar flares

This paper discusses the relationship between some characteristics of microwave type IV radio bursts and solar cosmic ray protons of MeV energy. It is shown that the peak flux intensity of those bursts is almost linearly correlated with the MeV proton peak flux observed by satellites near the Earth and that protons and electrons would be accelerated simultaneously by a similar mechanism during the explosive phase of solar flares. Brief discussion is given on the propagation of solar cosmic rays in the solar envelope after ejection from the flare regions.

Nonthermal Turbulent Heating in the Solar Envelope

view Abstract Citations (12) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Nonthermal Turbulent Heating in the Solar Envelope Papadopoulos, Konstantinos Abstract It is shown that MHD pulses, in the form of fast magnetosonic waves or solitons, can produce a strong electron-ion coupling capable of maintaining T T0 in the solar envelope (r < 30 R0). The mechanism producing the nonthermal heating is the fluid-like modified two-stream instability, which, since it is essentially independent of the ratio and the value of p, becomes a prime candidate for the anomalous collisions required by the fluid models inside r 30 R0, in order to explain the dominant features of the solar-wind flow. Subject headings: corona, solar - hydromagnetics - solar wind - plasmas Publication: The Astrophysical Journal Pub Date: February 1973 DOI: 10.1086/151930 Bibcode: 1973ApJ...179..939P full text sources ADS |

The Five-Minute Oscillations on the Solar Surface

view Abstract Citations (316) References (17) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Five-Minute Oscillations on the Solar Surface Ulrich, Roger K. Abstract The acoustic properties of the subphotospheric layers are examined. It is shown that standing acoustic waves may be trapped in a layer below the photosphere. These standing waves may exist only along discrete lines in the diagnostic diagram of horizontal wavenumber versus frequency. The positions of these lines are derived from a modal analysis of the solar envelope. The lines for the fundamental mode and the first-overtone mode pass through the centers of the two peaks observed by Frazier. An examination of the energy balance of the oscillations shows that they are overstable. When they are assigned an amplitude of 0.2 km , they generate about (5-7) x 106 ergs . This power output suggests that the dissipation of the 5-minute oscillations above the temperature minimum is responsible for heating the chromosphere and corona. Publication: The Astrophysical Journal Pub Date: December 1970 DOI: 10.1086/150731 Bibcode: 1970ApJ...162..993U full text sources ADS | Related Materials (1) Commentary: 1999ApJ...525C1199B

Spectroscopic Observations of Differential Rotation With Height in the Solar Envelope

AbstractThe problem of determining the spectroscopic rotation of the sun is not one of instrumental sensitivity, which can be about 10 m/sec, but rather of abstracting the rotational component from the hierarchy of long-lived surface currents. Required are continued daily observations over long periods of time and this task is not presently feasible at most observatories. Past observations have indicated a height gradient of rotational angular velocities. We have determined the photosphere and chromosphere angular velocity gradient from observations across the disk of C 5380 Å(h≃ -250 km), CN 3882 Â (h ≃ 0 km), Ca+K3(h≃+ 5000 km), together with other Fraunhofer lines of intermediate origin while K-line spectra of prominences are used to sense the chromosphere-corona interface.

Absolute Solar Spectra 35–55 Microns II Theoretical Spectra for Altitude Range 15–30 km

Transmission functions are applied to a model representing the atmosphere for specified pressures and concentrations of absorbing gases corresponding to total atmospheric paths above altitudes of 15 km to 30 km and zenith angles of 0° and 40°. The computed transmittance is then applied to the solar spectral radiance envelope curve obtained in Part I to give solar spectra with 0.02-μ resolution in the range 3.5–5.5 μ.

High Altitude Atmospheric Transmission Measurements

This program was designed to obtain atmospheric transmission data from high altitude, in the 1 to 12 μ region, and to measure the solar radiation spectrum in the same region. Mixing ratios, water to total atmospheric gases, at 11 km (35,000 ft) and 20 km (65,000 ft) altitude indicate a stratum of humid air above 20 km. A ground spectrum, an 11 km spectrum, and a 20 km spectrum are compared to a solar envelope constructed from available data.

Solar radiation in the radio spectrum - I. Radiation from the quiet sun

The theory of the emission of thermal radiation from the solar envelope at radio-frequencies is worked out in detail. The Lorentz theory of absorption is used in conjunction with Kirchhoff’s law to derive the effective temperature of the various regions of the solar disk over the radio spectrum. A maximum effective temperature approaching 10 6 °C is found in the vicinity of 1 m. wave-length. Limb brightening occurs at centimetre wave-lengths. It is shown that Gaunt’s quantum mechanical expression for free-free emission yields results almost identical with the classical treatment, provided Chapman and Cowling’s expression for the collision frequency in a fully ionized gas is used in the latter treatment. It is suggested that it may be preferable to treat problems of solar and galactic radio noise by classical methods, particularly when the refractive index of the medium departs appreciably from unity.

Origin of Solar 'Static'

RECENTLY, Martyn1 wrote : “It is natural first to seek an explanation [of the origin of solar static] in terms of the kind of electrical discharge which causes the familiar terrestrial radio noise, namely, lightning flashes ; but it is difficult, if not impossible, to see how such discharges could be produced in the always highly ionized solar envelope”. The explanation to which Martyn raises objection is, in fact, one which I have myself put forward recently2. His objection, however, does not arise on the general theory which I have advanced elsewhere3. Far from preventing the existence of electric fields at the sun's surface, the thermal conditions there are themselves the result of electrical discharges. These discharges occur in the radial electric field set up by the emission of highly charged atoms from nuclear reactions in the sun's interior.

Circular Polarization of Solar Radio Noise

IN two recent communications to these columns1,2 we have reported the existence of powerful radio emissions in the 5-metre wave-length band from sunspot areas. Since such radio waves must travel through regions of considerable ionization in escaping from the sun, it occurred to us that the magneto-ionic theory of radio wave propagation3, which has proved so useful in elucidating phenomena in the terrestrial ionosphere, would be applicable in the case of the corresponding solar envelope. According to, this theory, characteristic polarizations are imposed on radio waves in their transmission through an ionized medium under the influence of a magnetic field, due either to differential absorption of the oppositely polarized magneto-ionic components or to the suppression of one component by electron limitation. Such effects are most pronounced if the radio wave frequency in question is either of the same order as, or less than, the electronic gyro-frequency determined by the imposed magnetic field. There is also the possibility that the noise itself has a magneto-gyric (electronic) origin.

Ultra‐violet solar radiation

The work of Abbot and his colleagues has shown that the radiation from the Sun varies and that the variation is most pronounced in the violet end of the spectrum. Whether we regard the variation as due to a change in temperature, a variable transmission of the outer solar envelope or a variable emissivity, this effect on the ultra‐violet is to be expected. Dobson in England using a photographic method, measured the ultra‐violet solar radiation transmitted by silver‐films on thirty‐four days in 1922, and found variations as great as five hundred per cent, and a standard daily range of thirty per cent outside the atmosphere. It occurred to the writer that the uncertainties of photography could be eliminated by measuring the radiation directly with a thermo‐couple; that disturbances of the atmosphere could be made differential by standardizing the ultra‐violet with the green region of the spectrum which varies only slightly; that the accuracy could be increased by making the apparatus automatic, which would permit an increase in the number of observations, and that the combination of this procedure with the favorable atmospheric conditions on Mount Wilson might lead to results of some interest.

Ultra-Violet Solar Radiation

The work of Abbot and his colleagues has shown that the radiation from the Sun varies and that the variation is most pronounced in the violet end of the spectrum. Whether we regard the variation as due to a change in temperature, a variable transmission of the outer solar envelope or a variable emissivity, this effect on the ultra-violet is to be expected. Dobson in England using a photographic method, measured the ultra-violet solar radiation transmitted by silver-films on thirty-four days in 1922, and found variations as great as five hundred per cent, and a standard daily range of thirty per cent outside the atmosphere. It occurred to the writer that the uncertainties of photography could be eliminated by measuring the radiation directly with a thermo-couple; that disturbances of the atmosphere could be made differential by standardizing the ultra-violet with the green region of the spectrum which varies only slightly; that the accuracy could be increased by making the apparatus automatic, which would permit an increase in the number of observations, and that the combination of this procedure with the favorable atmospheric conditions on Mount Wilson might lead to results of some interest.